Vehicular glass assemblies, particularly large movable van windows, often have one or more components such as hinges or clevises affixed to the glass panel. When such assemblies are installed in vehicles, the assembly components and affixment means are often subjected to highly localized loads including dynamic loads, resulting from road vibration and opening and closing of the glass assembly. Therefore, original equipment manufacturer (OEM) specifications, utilized by most automotive companies to ensure that components supplied are suitable for use in vehicles, have required that components affixed to vehicular glass be able to withstand relatively high loads and severe vibration. At present, the primary method by which to attach such components that meets OEM specifications is by mechanical affixment of the component, e.g., hinge or clevis, to the glass panel. Typically, such mechanical affixment requires forming one or more holes through the glass panel, inserting component fasteners, and then mechanically attaching the components to the glass via the fasteners. In some applications involving wrap-around hinges, the formation of holes is avoided by positioning the hinge over a portion of the outer edge of the glass panel and clamping the hinge to the panel by use of one or more fasteners.
Neither of these approaches is entirely satisfactory in that each is relatively expensive due to the significant amount of time and labor involved in drilling holes and inserting or clamping fasteners, notwithstanding the cost of the additional fasteners or clamps. Moreover, mechanical affixment of components to glass necessitates the use of a mounting member or fastener on the opposite side of the glass from which the component is residing. This is undesirable in terms of both cosmetic appeal and resulting aerodynamics if the mounting member or fastener projects from the exterior surface of the glass panel, particularly as automotive designers strive to improve vehicle aesthetics and aerodynamics such as is useful, for example, in flush glazing of vehicles.
Single-component urethane adhesives have been used as a window fixing adhesive to bond glass panels to an automobile body and for mounting lass mounting members. Such adhesives have also been used to attach studs, mirror mounting buttons, and the like to automotive glazing. As known to those skilled in the art, a single-component urethane adhesive utilizes a highly prepolymerized isocyanate-rich urethane (typically formed from reacting an isocyanate with a polyol) typically mixed with an isocyanate (usually a polymeric isocyanate). The adhesive initially “sets” and finally “cures” when exposed to moisture in the air which causes the isocyanate moieties to cross-link with the urethane prepolymer(s) via a urea cross-linking reaction. The process usually requires several hours for the adhesive to set and several days for a complete cure to be effected. Such long set and cure periods present challenges when used in high production rate processes, such as automobile module assembly, because curing of the system typically requires temporary or permanent fixturing of the glass or mounting member(s), thus adding cost and extraneous equipment to the manufacturing process. Curing of single-component urethanes may also require the use of humidity chambers, and in certain cases, the addition of heat to accelerate bonding which further adds cost, time, inventory, handling, and complexity to the manufacturing process.
Additionally, single-component urethanes, when employed as a window fixing adhesive, limit the range of applications of the resulting vehicular glass assembly. Such bonded assemblies are typically utilized so that only a relatively low load or stress occurs in the adhesive material or at the bond between the glass and the adhesive. Cured, single-component urethanes often cannot withstand highly localized loads or high stresses, particularly at extreme environmental conditions such as high temperatures, exposure to moisture, and vibration.
In another respect, single-component urethanes are not entirely satisfactory for forming bonded glass assemblies, since single-component urethanes may induce stresses on the glass upon curing. Such stress increases the likelihood of glass fracture upon application of additional external forces or other stresses thereby rendering such adhesives unsatisfactory for forming joints in dynamic load-bearing applications in a vehicle.
Two-component urethane adhesive systems have been used for bonding fixed vehicle windows, such as windshields, for certain after-market applications. The term “two-component system” refers to urethane adhesive systems in which the isocyanate and polyol components are kept separate from one another until just prior to bonding at which time those components are mixed together and applied on the appropriate surfaces for bonding. Upon mixing the two components, a full urethane polymerization reaction occurs. The problems encountered with two-component systems currently recommended for after-market applications are that such systems are still relatively slow, typically requiring about 1 hour to set and about 3 days to cure, and result in a relatively weak initial bond that does not reach full cure properties for days.
At present, the automotive industry lacks a method for securely attaching load-bearing components, and particularly dynamic load-bearing components, to glass window panels that meets OEM requirements, is aesthetically appealing, is economical, and which avoids the numerous drawbacks of utilizing mechanical attachment methods. Prior art suggestions and attempts have not resulted in any commercially successful two-component urethane adhesive that is suitable for bonding hardware components to glass panels for use in vehicles, particularly for bonding hinges, clevises, and similar attachments typically utilized in large area movable windows, such as those found in automobiles, minivans, sport utility vehicles, trucks, and buses.